An antireflection film is generally disposed on the outermost surface of a display so as to reduce reflectance using the principle of optical interference from the viewpoint of preventing a lowering in contrast and a lowering in visibility caused by external light reflection and reflected image catching in display devices such as cathode-ray tube displays (CRTs), plasma display panels (PDPs), electroluminescent displays (ELDs), and liquid crystal display devices (LCDs).
The antireflection film can be generally prepared by forming a low-refractive index layer having a proper thickness and having a lower refractive index than the support on a support.
In recent years, regarding antireflection films, a trend toward expansion of display markets and an increase in size of displays has particularly led to a demand for a) increased area and homogeneous properties, b) lowered price, and c) improved scratch resistance. The homogeneous properties referred to in the above demand a) means that the planar appearance within the whole display part in the display is homogeneous and there is no variation in properties of the antireflection film typified by reflectance.
Regarding the lowered price in the above demand b), not only a reduction in cost of materials for constituting the antireflection film but also an improvement in productivity of the antireflection film is very important. In order to improve the productivity, the development of a technique which can realize high-speed coating production without sacrificing the homogeneity of planar appearance and other various properties (without sacrificing product yield) has been indispensable. In the production of the antireflection film, as compared with films formed by a dry process typified by vapor deposition and sputtering, basically, the formation of a film by a wet coating process is disadvantageous in that, although there is a potential that low-cost production could be realized, high-speed coating is likely to cause uneven flow, uneven drying, or coating streaks by entrained air and drying air, and, consequently, is likely to cause uneven layer thickness. Thus, maintaining and improving the required homogeneity of the antireflection film is very difficult, leading to a serious problem.
Regarding the scratch resistance in the above demand c), due to the expansion of display markets, the opportunity for rough handling, which has not hitherto been found frequently, is expected to be increased. Accordingly, improving the scratch resistance of the antireflection film used in the outermost surface of the display is becoming a very important task to be attained.
It is known that improving the leveling properties of the coating film is effective in improving uneven layer thickness involved in the above higher-speed coating. The addition of a surfactant (a leveling agent) to a coating liquid has been proposed as one means for improving the leveling properties. This proposal is based on such a mechanism that the addition of a surfactant to a coating liquid causes lowered surface tension to realize improved wettability of an object to be coated with the coating liquid, which then reduces or lowers a surface tension change in the course of coating film formation and consequently prevents flow and convection of the liquid to improve the homogeneity of the film (Kotinguyo Tenkazai No Saishin Gijutsu (Advanced technology for additives for coating), edited by Haruo Kiryu, CMC, 2001).
Japanese Patent Laid-Open No. 249706/2002 discloses a case where excellent leveling properties are realized using a fluoropolymer-type leveling agent containing a perfoluoroalkyl group having 6 or less carbon atoms. When coating is carried out using the above fluoro leveling agent, the leveling effect is likely to provide homogeneous planar appearance. Since, however, the surface free energy of a coating film after drying is reduced, stacking of a low-refractive index layer indispensable to the antireflection film onto the coating face weakens the adhesion at the interface. As a result, breaking is likely to occur by external force, disadvantageously leading to a problem that only antireflection films having poor scratch resistance can be produced.
Japanese Patent Laid-Open No. 53878/2003 discloses a method which comprises adding a silicone leveling agent to a coating liquid for high-refractive index layer formation, forming a high-refractive index layer using the coating liquid, and further forming a low-refractive index layer, whereby a leveling agent layer is formed between the high-refractive index layer and the low-refractive index layer. In Japanese Patent Laid-Open No. 53878/2003, however, there is no description on an improvement in scratch resistance. Japanese Patent Laid-Open No. 53878/2003 aims to reduce a difference in refractive index from the high refractive index layer by using a silicone leveling agent having a high refractive index and thus to attain the effect of reducing an increase in reflectance found in the conventional silicone leveling agent. This technique is different from the present invention in purpose. Further, the addition of a silicone leveling agent to an anti-dazzling layer followed by stacking of a low-refractive index layer indispensable to the antireflection film, as with the above case, is disadvantageous in that, although the leveling effect is likely to provide homogeneous planar appearance, the adhesion at the interface of the anti-dazzling layer and the low-refractive index layer is so low that the scratch resistance is significantly lowered, or the low-refractive index layer cannot be coated well on the concave-convex parts in the anti-dazzling layer,-leading to problems such as cissing.
An antireflection film is generally disposed on the outermost surface of a display so as to reduce reflectance using the principle of optical interference from the viewpoint of preventing a lowering in contrast and a lowering in visibility caused by external light reflection and reflected image catching in display devices such as cathode-ray tube displays (CRTs), plasma display panels (PDPs), electroluminescent displays (ELDs), and liquid crystal display devices (LCDs).
The antireflection film can be generally prepared by forming a low-refractive index layer having a proper thickness and having a lower refractive index than the support on a support.
In recent years, regarding antireflection films, a trend toward expansion of display markets and an increase in size of displays has particularly led to a demand for a) increased area and homogeneous properties, b) improved surface hardness and scratch resistance, c) a cost reduction such as high-speed coating and the like. The homogeneous properties referred to in the above demand a) means that there is no variation in properties of the antireflection film typified by reflectance and, in addition, the antireflection film has a homogeneous, abnormality-free planar appearance within the whole display part in the display.
Regarding the surface hardness and scratch resistance in the above demand b) as well, due to the expansion of display markets, the opportunity for rough handling, which has not hitherto been found frequently, is expected to be increased. Accordingly, improving the surface hardness and scratch resistance of the antireflection film used in the outermost surface of the display is becoming a very important task to be attained.
Regarding the high-speed coating in the above demand c), it is known that improving the leveling properties of the coating film is effective in suppressing uneven layer thickness involved in the above higher-speed coating. The addition of a surfactant (a leveling agent) to a coating liquid has been proposed as one means for improving the leveling properties.
Japanese Patent Laid-Open No. 249706/2002 discloses a case where excellent leveling properties are realized using a fluoropolymer-type leveling agent containing a perfoluoroalkyl group having 6 or less carbon atoms. When coating is carried out using the above fluoro leveling agent, the leveling effect is likely to provide homogeneous planar appearance. Since, however, the surface free energy of a coating film after drying is reduced, stacking of a low-refractive index layer indispensable to the antireflection film onto the coating face weakens the adhesion at the interface. As a result, breaking is likely to occur by external force, disadvantageously leading to a problem that only antireflection films having poor scratch resistance can be produced.
Japanese Patent Laid-Open No. 53878/2003 discloses a method which comprises adding a silicone leveling agent to a coating liquid for high-refractive index layer formation, forming a high-refractive index layer using the coating liquid, and further forming a low-refractive index layer, whereby a leveling agent layer is formed between the high-refractive index layer and the low-refractive index layer. In Japanese Patent Laid-Open No. 53878/2003, however, there is no description on an improvement in scratch resistance. Japanese Patent Laid-Open No. 53878/2003 aims to reduce a difference in refractive index from the high refractive index layer by using a silicone leveling agent having a high refractive index and thus to attain the effect of reducing an increase in reflectance found in the conventional silicone leveling agent. This technique is different from the present invention in purpose. Further, the addition of a silicone leveling agent to an anti-dazzling layer followed by stacking of a low-refractive index layer indispensable to the antireflection film, as with the above case, is disadvantageous in that, although the leveling effect is likely to provide homogeneous planar appearance, the adhesion at the interface of the anti-dazzling layer and the low-refractive index layer is so low that the scratch resistance is significantly lowered, or the low-refractive index layer cannot be coated well on the concave-convex parts in the anti-dazzling layer, leading to problems such as cissing.
In order to provide a surface hardness to be satisfied by an antireflection film, an effective method is to provide a hardcoat layer using, as a binder, a light transparent ionizing radiation curing-type polyfunctional resin of an acrylic resin (for example, an anti-dazzling hardcoat layer to which surface light scattering properties have been imparted by surface concaves and convexes) to a given or larger thickness.
The present inventors, however, have confirmed that this technique involves a latent problem that leads to a lowering in a commercial value, that is, that the application of, for example, ultraviolet light (UV) at an intensity necessary for ensuring the adhesion between the triacetylcellulose film as a support and the layer in direct contact with the triacetylcellulose film causes acrylic resin-derived uneven curing shrinkage, and, in particular, in the hardcoat layer and the anti-dazzling hardcoat layer of which the thickness has been increased for ensuring the surface hardness, the surface of the antireflection film becomes a folded and cockled state, that is, an abnormal appearance occurs.